EP0624907A2

EP0624907A2 - Semiconductor device, heterojunction bipolar transistor, and high electron mobility transistor

Info

Publication number: EP0624907A2
Application number: EP94106749A
Authority: EP
Inventors: Shigekazu C/O Mitsubishi Denki K.K. Izumi
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1993-05-10
Filing date: 1994-04-29
Publication date: 1994-11-17
Also published as: EP0624907A3; JPH06318606A; US5459331A

Abstract

A semiconductor device includes a lamination structure comprising a GaAs layer and an InGaAs layer (32b) grown on the GaAs layer, through which operating current flows in the thickness direction of the InGaAs layer. The InGaAs layer (32b) includes a plurality of very thin GaAs layers (1), through which most of the operating current passes due to tunnel effect, inserted into the InGaAs layer (23b) at prescribed intervals larger than a critical thickness that maintains a pseudomorphic state of an InGaAs crystal grown on a GaAs crystal. Therefore, segregation of In atoms, i.e., unfavorable move of In atoms in the growing InGaAs crystal toward the surface, which occurs remarkably when the InGaAs layer is grown at a high temperature, or elimination of In atoms can be suppressed by the very thin GaAs layers, so that the InGaAs layer can be grown on the GaAs layer at a high temperature without degrading the surface mohology of the InGaAs layer. As the result, an InGaAs layer with improved surface mohology, reduced contact resistivity and sheet resistivity, and improved uniformities of these resistivities in a wafer can be grown on the GaAs layer.

Description

FIELD OF THE INVENTION

The present invention relates to semiconductor devices having heterojunctions, such as HBTs (Heterojunction Bipolar Transistors) and HEMTs (High Electron Mobility Transistors) and, more particularly, to an improvement of surface mohology of a lattice mismatch crystal structure comprising a base semiconductor layer and a semiconductor layer having a lattice constant different from that of the base layer and grown on the base layer to a thickness larger than the critical thickness.

BACKGROUND OF THE INVENTION

Figure 10 is a perspective view illustrating a prior art HBT. In the figure, an HBT 200 includes a GaAs substrate 201 which is produced by a liquid encapsulated czochralski method (hereinafter referred to as LEC). A buffer layer 201a about 1 µm thick is disposed on the GaAs substrate 201. The buffer layer 201a comprises an intrinsic type (hereinafter referred to as i type) GaAs/AlGaAs superlattice layer about 800 nm thick and an i type GaAs layer about 200 nm thick layer. A collector contact layer 211 is disposed on the buffer layer 201a. A collector layer 212 is disposed on a center part of the collector contact layer 211, and collector electrodes 210 comprising three layers of AuGe/Ni/Au are disposed on the collector contact layer 211 at opposite sides of and spaced apart from the collector layer 212. The collector contact layer 211 comprises an n⁺ type GaAs layer having a thickness of 500 nm and a dopant concentration of 5 x 10¹⁸ cm⁻³, and the collector layer 212 comprises an n type GaAs layer having a thickness of 500 nm and a dopant concentration of 5 x 10¹⁶ cm⁻³.
A p⁺ type AlGaAs base layer 221 is disposed on the collector layer 212. The base layer 221 has a thickness of 100 nm and a dopant concentration of 1 ∼ 4 x 10¹⁹ cm⁻³. An n type AlGaAs emitter layer 231 is disposed on a center part of the base layer 221. The emitter layer 231 has a thickness of 150 nm and a dopant concentration of 5 x 10¹⁷ cm⁻³. Base electrodes 220 comprising three layers of Ti/Ni/Au are disposed on the base layer 221 at opposite sides of and spaced apart from the emitter layer 231. In the AlGaAs base layer 221, the ratio of AlAs mixed crystal gradually increases upward from 0 to 0.1. The emitter layer 231 comprises three AlGaAs layers, i.e., an AlGaAs layer grown on the base layer 221 with the AlAs mixed crystal ratio gradually increasing upward from 0.1 to 0.3, an Al_0.3Ga_0.7As layer grown thereon to a prescribed thickness, and an AlGaAs layer grown thereon with the AlAs mixed crystal ratio gradually decreasing upward from 0.3 to 0.
An n⁺ type InGaAs emitter contact layer is disposed on the emitter layer 231. The emitter contact layer 232 has a thickness of 100 nm thick and a dopant concentration of 4 x 10¹⁹ cm⁻³. An emitter electrode 230 comprising three layers of Ti/Mo/Au is disposed on the emitter contact layer 232. In the emitter contact layer 232, the ratio of InAs mixed crystal gradually increases upward from 0 to 0.5. Since the emitter contact layer 232 is disposed for reducing the contact resistance between the emitter layer 231 and the emitter electrode 230, InGaAs having a low contact resistance with the emitter electrode 230 and a low sheet resistivity is employed as a material of the emitter contact layer 232. Insulating regions 202 are disposed contacting the opposite sides of the collector contact layer 211 and reaching the surface of the buffer layer 201a. Insulating regions 203 are disposed contacting opposite sides of the base layer 221 and the collector layer 212.
A method for manufacturing the HBT of figure 10 is illustrated in figures 11(a)-11(f) and 12(a)-12(f).
Initially, an i type GaAs layer 201a about 1 µm thick, an n⁺ type GaAs layer 211 about 500 nm thick, and an n type GaAs layer 212a about 500 nm thick are successively grown on the GaAs substrate 201. Thereafter, a p⁺ type AlGaAs layer 221a is epitaxially grown on the n type GaAs layer 212a to a thickness of 100 nm while gradually increasing the ratio of AlAs mixed crystal from 0 to 0.1.
Then, an AlGaAs layer is epitaxially grown on the p⁺ type AlGaAs layer 221a in the following manner. That is, in the initial stage of the epitaxial growth, the AlGaAs layer is grown while gradually increasing the AlAs mixed crystal ratio from 0.1 to 0.3 until a prescribed thickness is achieved and, thereafter, the AlGaAs layer is grown while maintaining the AlAs mixed crystal ratio at 0.3 until a prescribed thickness is achieved and, finally, the AlGaAs layer is grown while gradually decreasing the AlAs mixed crystal ratio from 0.3 to 0, completing an n type AlGaAs graded layer 231a having a thickness of 150 nm.
Then, an n⁺ type InGaAs layer is epitaxially grown on the AlGaAs graded layer 231a while gradually increasing the InAs mixed crystal ratio from 0 to 0.5, forming an n⁺ type InGaAs graded layer 232a having a thickness of 100 nm (figure 11(a)).
Thereafter, using a first photoresist film 205a of a prescribed pattern as a mask, proton ions are implanted reaching the boundary between the n type GaAs layer 212a and the n⁺ type GaAs layer 211, forming first insulating regions 203 (figure 11(b)). Then, using a second photoresist film 205b as a mask, proton ions are implanted reaching into the buffer layer 201a, forming second insulating regions 202 (figure 11(c)).
Thereafter, a dummy emitter 241 comprising an insulating film is formed on a part of the InGaAs layer 232a, and a mask pattern 242 comprising WSi or Au and wider than the dummy emitter 241 is formed on the dummy emitter 241 (figure 11(d)). The mask pattern 242 is used in subsequent lift-off process. Using the dummy emitter 241 as a mask, the n⁺ InGaAs layer 232a is etched to form an emitter contact layer 232 (figure 11(e)).
Then, a photoresist film is deposited and patterned to form a third photoresist pattern 205c having an aperture around the dummy emitter 241. Using the photoresist pattern 205c as a mask, the n type AlGaAs layer 231a is selectively etched to from an emitter layer 231. Thereafter, a base metal layer 220a is deposited over the entire surface (figure 11(f)), and the photoresist pattern 205c and overlying portions of the base metal layer 220a are removed by a lift-off technique, leaving base electrodes 220 (figure 12(a)).
Then, a fourth photoresist film 205d is deposited over the entire surface so that the upper surface of the photoresist film 205d reaches the lower surface of the mask pattern 242. Using the photoresist film 205d as a mask, the dummy emitter 241, the mask pattern 242, and the base metal 220a are removed (figure 12(b)).
Then, an emitter metal 230a is deposited over the entire surface (figure 12(c)), and the fourth photoresist film 205d and overlying portions of the emitter metal 230a are removed by a lift-off technique, leaving an emitter electrode 230 (figure 12(d)).
Thereafter, two grooves are formed penetrating through portions of the n type AlGaAs graded layer 231a, p⁺ type AlGaAs layer 221a, and n type GaAs layer 212a at opposite sides of the emitter electrode 230, and collector electrodes 210 are formed in the respective grooves (figure 12(e)).
Finally, a surface protection film 206 is formed over the entire surface, and portions of the surface protection film 206 on the collector electrodes 210 are removed to form apertures 206a. Then, a wiring layer 207 is formed on the surface protection film 206 so that the wiring layer 207 is in contact with the collector electrodes 210 through the apertures 206a of the surface protection film 206 (figure 12(f)). The spaced apart portions of the wiring layer 207 in contact with the respective collector electrodes 210 at opposite sides of the collector layer 212 are connected to each other by an air bridge wiring 207a.
A problem in manufacturing the HBT resides in the process of growing the n⁺ type In_yGa_1-yAs layer 232a (emitter contact layer 232) on the n type Al_yGa_1-yAs (y: 0.1 - 0.3 - 0) graded layer 231a (emitter layer 231), i.e., on a GaAs layer.
A description is given of the problem in the growth of an In_yGa_1-yAs layer (y: 0 ∼ 1.0) on a GaAs layer, which layers have different lattice constants, to a prescribed thickness that provides a sufficiently low contact resistance.
Figures 13(a)-13(b) are diagrams for explaining a GaAs crystal lattice and an InAs crystal lattice, respectively, and figure 13(c) is a graph illustrating lattice constant (a) vs. energy band gap (EG) characteristics of AlGaAs and InGaAs which are typical III-V mixed crystal semiconductors. In figure 13(a), a unit crystal lattice of GaAs (hereinafter referred to as GaAs crystal) 10a comprises As atoms 11 and Ga atoms 12 and has a lattice constant of 5.6535 Å. In figure 13(b), a unit crystal lattice of InAs (hereinafter referred to as InAs crystal) 10b comprises As atoms 11 and In atoms 13 and has a lattice constant of 6.0584 Å.
The III-V compound semiconductors include mixed crystal semiconductors comprising three kinds of elements, such as AlGaAs and InGaAs, other than the above-described mixed crystal semiconductors comprising two kinds of elements. In these ternary compound semiconductors, the energy band gap (EG) can be continuously varied by varying the AlAs or InAs mixed crystal ratio. Especially AlGaAs is favorable as a constituent of a semiconductor device because the lattice constant of AlGaAs is not varied with the variation of the AlAs mixed crystal ratio. However, the energy band gap of AlGaAs is larger than that of GaAs. On the other hand, the energy band gap of InGaAs is smaller than that of GaAs. Therefore, for the purpose of reducing the resistance of a semiconductor layer included in a device, InGaAs has an advantage over GaAs.
However, the lattice constant of InGaAs having an InAs mixed crystal ratio of 1, i.e., the lattice constant a₂ of the InAs crystal, is different from the lattice constant a₁ of the GaAs crystal by 7.2 %. Therefore, a coherent single crystal is not attained due to dislocation when an In_yGa_1-yAs layer is grown on a GaAs layer. In figure 14(a), a crystal structure of a monocrystalline In_0.5Ga_1-0.5As layer 20b (lattice constant a₁₂) is compared with a crystal structure of a monocrystalline GaAs layer 20a (lattice constant a₁).
As shown in figure 14(b), when the thickness T₁ of the InGaAs layer 20b is very thin, the lattice mismatch generated by the dislocation of the crystal lattice is eased by the strain of the crystal lattice, and a pseudomorphic crystal structure is attained. Therefore, the monocrystalline InGaAs layer 20b can be grown on the GaAs layer 20a.
On the other hand, in the crystal growth of InGaAs, segregation of In atoms, i.e., escape of In atoms in the InGaAs layer 20b toward the surface of the layer (figure 15(a)), or elimination of In atoms occurs, resulting in a crystal defect that facilitates the dislocation. If the thickness of In_yGa_1-yAs whose lattice constant does not match with the lattice constant of GaAs exceeds the critical thickness T₀, a dislocation 22 is produced by the crystal defect, and the strain of the InGaAs crystal is relaxed. As a result, the pseudomorphic structure is destroyed in a part 21b₁ exceeding the critical thickness T₀ of the In_yGa_1-yAs layer 21b grown on the GaAs layer 20a as shown in figure 15(b), and a uncoherent or polycrystalline InGaAs is grown. Further, in such a crystal growth, the segregation of In atoms and the elimination of In atoms are facilitated, so that a favorable surface mohology of the InGaAs layer is not attained.
As shown in figure 16, the critical thickness T₀ of the InGaAs layer decreases with an increase in the InAs mixed crystal ratio (y) of the InGaAs layer. For example, the critical thicknesses at the InAs mixed crystal ratios of 0.15 and 0.2 are 250 Å and 200 Å, respectively. If the crystal thickness is exceeded, the pseudomorphic crystal growth is impossible.
Therefore, if an InGaAs layer grown on a GaAs layer to a thickness over the critical thickness T₀ is applied to an HBT, fine patterns of insulating film and conductive film cannot be formed on the InGaAs layer with high reproducibility because of the rough surface of the InGaAs layer.
This problem will be described concretely.
For example, when the dummy emitter 241 is formed by patterning an insulating film in the above-described production process of HBT, if the insulating film 241a and the metal layer 242a are formed on the n⁺ type InGaAs layer 232a of rough surface mohology as shown in figure 17(a), the rough surface mohology of the InGaAs layer 232a adversely affects the surface mohologies of the insulating film 241a and the metal layer 242a.
If a photoresist film is deposited and patterned on the metal layer 242a, the side surface 215 of the patterned photoresist film 205 is rough due to the irregular reflection of exposure light (figure 17(a)). When the metal layer 242a and the insulating film 241a are patterned using the photoresist pattern 205 as a mask, the side surfaces 42 and 41 of the patterned metal layer 242 and the patterned insulating dummy emitter 241, respectively, are rough because of the rough side surface 215 of the photoresist pattern 205 and the rough surface mohologies of the metal layer 242a and the insulating film 241a (figure 17(b)). Therefore, a fine dummy emitter is not attained, which makes it difficult to form a fine emitter.
While in the above-described production process of HBT the space between the emitter layer 231 and the base electrode 220 depends on the width of the over-hanging portion of the metal film 242 on the dummy emitter 241, (figure 11(d)), a side wall may be interposed between the emitter layer 231 and the base electrode 220. In this case, however, it is difficult to form the side wall.
Figure 18 is a sectional view illustrating an HBT including side walls 235 interposed between the emitter layer 231 and the respective base electrodes 220. In figure 18, the same reference numerals as in figure 10 designate the same or corresponding parts. These side walls 235 are formed contacting the opposite side surfaces of the GaAs emitter layer 231 and the InGaAs emitter contact layer 232 after the formation of these layers and serve as a mask for patterning the emitter electrode 230. After the device is completed, these side walls 235 serve as insulators between the respective base electrodes 220 and the emitter electrode 230. Further, since the portions of the base layer 221 between the emitter layer 231 and the respective base electrodes 220 are covered with the side walls 235, surface recombination current at these portions is reduced.
In the production of the HBT shown in figure 18, if the surface mohology of the InGaAs layer that is to be the emitter contact layer 232 is rough, a photoresist film formed on the InGaAs layer for patterning of the emitter layer and the emitter contact layer has rough side surfaces, with a result that the side walls are not favorably formed on the side surfaces of the emitter layer and the emitter contact layer.
As described above, in the crystal structure formed by growing an InGaAs layer on a GaAs layer which layers have different lattice constants, a favorable surface mohology of the InGaAs layer is not achieved. If such a crystal structure is applied to a device, such as HBT, it is difficult to form a fine pattern on the InGaAs layer.
Meanwhile, Japanese Published Patent Application No. Hei. 3-280419 discloses a method for growing an n type InGaAs layer on a GaAs layer, which layers have different lattice constants. In this prior art, in order to avoid the degradation of the surface mohology of the InGaAs layer, the InGaAs layer is grown at a low temperature. However, in the low temperature growth for improving the surface mohology, a crystal structure of good quality is not attained, whereby the contact resistivity between the semiconductor layer and the metal layer and the sheet resistivity unfavorably increase. In addition, uniformities of the contact resistivity and the sheet resistivity in a wafer are reduced.
Japanese Published Patent Application No. Hei. 4-72740 discloses an ohmic electrode comprising a superlattice layer disposed on a GaAs substrate and a metal layer disposed on the superlattice layer. The superlattice layer comprises alternating n type In_xGa_1-xAs layers and n type In_yGa_1-yAs layers, in which both of the indium-composition ratios x and y gradually increase upward while maintaining the relation of x < y, and the n type In_yGa_1-yAs layer is present at the top of the superlattice layer, whereby dislocation in the superlattice layer is suppressed and a low resistance ohmic contact is realized. In this structure, however, compositions of the n type In_xGa_1-xAs layer and the n type In_yGa_1-yAs layer must be precisely controlled. Furthermore, since the compositions of these layers are gradually varied, an interface having a large difference in lattice constants is not present in the superlattice layer. Therefore, the segregation of In atoms that causes the rough surface mohology is not effectively prevented.
Further, Japanese Published Patent Application No. Sho. 63-186416 discloses a compound semiconductor substrate comprising an Si substrate and a high electron mobility compound semiconductor layer, such as GaAs, grown on the Si substrate. The compound semiconductor layer comprises alternating In_0.3Ga_0.7As layers and GaAs layers which prevent a dislocation at the interface between the substrate and the compound semiconductor layer from spreading in the compound semiconductor layer, and the thickness of each layer of the alternating compound semiconductor layers is larger than the critical thickness, whereby the quality of the compound semiconductor layer is improved. In addition, the increase in the thickness improves the reproducibility. In the alternating compound semiconductor layers, however, the GaAs layer having an energy band gap larger than that of the In_0.3Ga_0.7As layer is as thick as 500 Å, so that the sheet resistivity of the alternating layers is significantly increased due to the GaAs layer. Therefore, this structure is unfavorable to an improvement of device characteristics.
Further, Japanese Published Patent Application No. Sho. 63-156356 discloses a pnp bipolar transistor including an emitter layer of a strained superlattice structure in which a plurality of p type InGaAs layers of large lattice constant and small energy band gap and a plurality of p type GaAs layers of small lattice constant and large energy band gap are alternatingly laminated, whereby the effective mass of holes in the emitter layer is reduced to realize an operating speed as high as that of an npn transistor.
However, in the strained superlattice structure, two layers of different lattice constants, i.e., the InGaAs layer and the GaAs layer, are in the pseudomorphic state, so that this structure is not for preventing the degradation of surface mohology of a lattice mismatch lamination structure.
As described above, in the conventional crystal structure in which an In_yGa_1-yAs layer is grown on a GaAs layer, if the epitaxial growth is carried out at a low temperature to improve the surface mohology, the contact resistivity and the sheet resistivity are unfavorably increased and the uniformities of these resistivities in a wafer are reduced. On the contrary, if the epitaxial growth is carried out at a high temperature to avoid these problems, segregation and elimination of In atoms adversely affect the surface mohology. Accordingly, the crystal growth of InGaAs on GaAs has been a problem in realizing a high-performance HBT.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor device including an In_yGa_1-yAs layer grown on a GaAs layer at a high temperature, in which the In_yGa_1-yAs layer has improved surface mohology, reduced contact resistivity and sheet resistivity, and high uniformities of these resistivities in a wafer.
Another object of the present invention is to provide a high-performance HBT including an InGaAs emitter contact layer grown on a GaAs emitter layer in which the InGaAs emitter contact layer has reduced contact resistivity between the emitter contact layer and an emitter electrode, reduced sheet resistivity, high uniformities of these resistivities in a wafer, and improved surface mohology that offers a fine emitter with high reproducibility.
Still another object of the present invention is to provide a high-performance HEMT including InGaAs source and drain contact layers grown on a GaAs emitter layer in which the InGaAs source and drain contact layers have reduced contact resistivity between the source and drain contact layers and source and drain electrodes, reduced sheet resistivity, high uniformities of these resistivities in a wafer, and improved surface mohology that offers fine source and drain with high reproducibility.
Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.
According to a first aspect of the present invention, a semiconductor device includes a lamination structure comprising a GaAs layer and an InGaAs layer grown on the GaAs layer, through which operating current flows in the thickness direction of the InGaAs layer. The InGaAs layer includes a plurality of very thin GaAs layers through which most of the operating current passes due to tunnel effect. These GaAs layers are inserted into the InGaAs layer at prescribed intervals wider than a critical thickness that maintains a pseudomorphic state of an InGaAs crystal grown on a GaAs crystal. Therefore, segregation of In atoms, i.e., unfavorable move of In atoms in the growing InGaAs crystal toward the surface, which occurs remarkably when the InGaAs layer is grown at a high temperature, or elimination of In atoms can be suppressed by the very thin GaAs layers, so that the InGaAs layer can be grown on the GaAs layer at a high temperature without degrading the surface mohology of the InGaAs layer. As the result, an InGaAs layer with improved surface mohology, reduced contact resistivity and sheet resistivity, and improved uniformities of these resistivities in a wafer can be grown on the GaAs layer. Furthermore, since the thickness of the very thin GaAs layer is selected so that most of the operating current passes through that layer, the insertion of the very thin GaAs layers in the InGaAs layer does not increase the resistance.
According to a second aspect of the present invention, in the above-describe semiconductor device, the InGaAs layer is disposed on an InGaAs graded layer that is grown on the GaAs layer while gradually increasing the ratio of InAs mixed crystal from 0 to a prescribed value to a thickness more than the critical thickness.
According to a third aspect of the present invention, in the above-describe semiconductor device, the InGaAs graded layer has the InAs mixed crystal ratio gradually increasing upward from 0 to 0.5, and the InGaAs layer disposed on the InGaAs graded layer is a contact layer with the InAs mixed crystal ratio of 0.5 on which an electrode is to be disposed.
According to a fourth aspect of the present invention, an HBT includes a monocrystalline GaAs emitter layer and an emitter contact layer disposed on the emitter layer. The emitter contact layer comprises an InGaAs graded layer which is grown on the GaAs emitter layer to a thickness more than the critical thickness of InGaAs while gradually increasing the InAs mixed crystal ratio from 0 to a prescribed value, and an InGaAs surface layer grown on the graded layer to a thickness more than the critical thickness while maintaining the InAs mixed crystal ratio at the prescribed value. The InGaAs surface layer includes a plurality of very thin GaAs layers which are inserted in the InGaAs surface layer at prescribed intervals larger than the critical thickness of InGaAs, through which most of the operating current passes due to tunnel effect. Therefore, the emitter contact layer can be grown at a high temperature without degrading the surface mohology. Since the surface mohology of the emitter contact layer is improved, fine patterns of insulating film and conductive film are formed on the emitter contact layer with high reproducibility. Furthermore, since the contact resistivity and the sheet resistivity of the emitter contact layer are suppressed, an HBT element with improved operating characteristics is achieved at good yield.
According to a fifth aspect of the present invention, an HEMT includes InGaAs source and drain contact layers disposed on an n type AlGaAs layer supplying two-dimensional electron gas via n type GaAs layers. Each of the n type InGaAs contact layers includes a plurality of very thin GaAs layers inserted into the InGaAs layer at prescribed intervals larger than the critical thickness of InGaAs, through which most of the operating current passes due to tunnel effect. Therefore, the InGaAs source and drain contact layers can be grown at a high temperature without degrading the surface mohology, whereby an HEMT having improved operating characteristics is achieved at good yield.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view illustrating an HBT in accordance with a first embodiment of the present invention.
Figure 2 is a sectional view illustrating a lamination structure of semiconductor layers included in the HBT of figure 1.
Figure 3 is a schematic diagram illustrating a molecular beam epitaxial apparatus for producing an emitter contact layer included in the HBT of figure 1.
Figure 4 is a graph illustrating growth temperature dependence of the surface mohology, the sheet resistivity Rs, and the deviation of the sheet resistivity Rs of a super-periods InGaAs layer included in the emitter contact layer according to the present invention, in comparison with those of an InGaAs emitter contact layer included in the prior art structure.
Figure 5 is a graph illustrating growth temperature dependence of the sheet carrier density Ns and the carrier mobility µ of the InGaAs emitter contact layer according to the present invention.
Figure 6 is a graph illustrating growth temperature dependence of the contact resistivity ρc of the InGaAs emitter contact layer according to the present invention.
Figures 7(a) and 7(b) are sectional views of samples used for measuring characteristics of the InGaAs with super-periods GaAs layer according to the present invention and characteristics of the InGaAs layer according to the prior art, respectively.
Figures 8(a) and 8(b) are schematic diagrams for explaining a mechanism in which the surface mohology of the InGaAs with super-periods GaAs layer is improved.
Figure 9 is a sectional view illustrating a HEMT in accordance with a second embodiment of the present invention.
Figure 10 is a perspective view illustrating an HBT according to the prior art.
Figures 11(a)-11(f) and 12(a)-(f) are sectional view illustrating process steps in a method for fabricating the HBT of figure 10.
Figures 13(a) and 13(b) are diagrams illustrating a GaAs crystal lattice and an InAs crystal lattice, respectively, and figure 13(c) is a graph illustrating relations between lattice constants of AlGaAs and InGaAs and energy band gaps thereof.
Figure 14(a) is a diagram illustrating crystal structures of an In_0.5Ga_1-0.5As layer and a GaAs layer, respectively, and figure 14(b) is a diagram illustrating the InGaAs layer and the GaAs layer in a pseudomorphic state.
Figures 15(a) and 15(b) are diagrams for explaining segregation of In atoms and dislocation, respectively.
Figure 16 is a graph illustrating critical thickness vs. InAs mixed crystal ratio to GaAs characteristics of an InGaAs layer grown on a GaAs layer.
Figures 17(a) and 17(b) are perspective views for explaining problems in a process of forming a dummy emitter in the prior art production method.
Figure 18 is a sectional view illustrating an HBT employing side walls for determining positions of base electrodes from an emitter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a perspective view illustrating an HBT in accordance with a first embodiment of the present invention. An HBT 100 of this first embodiment is different from the prior art HBT 200 shown in figure 10 only in that the emitter contact layer 132 disposed on the AlGaAs emitter layer 231 includes a super-periods structure 32b comprising five In_0.5Ga_0.5As layers 2 and four very thin GaAs layers 1 which are alternatingly laminated.
The lamination structure of semiconductor layers included in the HBT 100 will be described in more detail using figure 2. In figure 2, a buffer layer 201a is disposed on a LEC (Liquid Encapsulated Czochralski) GaAs substrate 201. The buffer layer 201a comprises a superlattice buffer layer 201b 8000 Å thick and an i type GaAs buffer layer 201c 2000 Å thick. The superlattice buffer layer 201b comprises forty i type GaAs layers each having a thickness of 50 Å and forty Al_0.3Ga_0.7As layers each having a thickness of 150 Å which are alternatingly laminated.
An n⁺ type GaAs collector contact layer 211 having a thickness of 5000 Å and a dopant concentration of 5 x 10¹⁸ cm⁻³ is disposed on the buffer layer 201a. An n type GaAs layer collector 212 having a thickness of 5000 Å and a dopant concentration of 5 x 10¹⁶ cm⁻³ is disposed on the n⁺ type GaAs layer 211. A p⁺ type AlGaAs base layer 221 having a thickness of 400 ∼ 1000 Å and a dopant concentration of 1 ∼ 4 x 10¹⁹ cm⁻³ is disposed on the collector layer 212. In the AlGaAs base layer 221, the ratio of AlAs mixed crystal is gradually varied upward, i.e., in the growth direction, from 0 to 0.1.
An emitter layer 231 is disposed on the base layer 221. The emitter layer 231 comprises an n type AlGaAs lower graded layer 31a in contact with the base layer 221, an n type Al_0.3Ga_0.7As center layer 31b, and an n type AlGaAs upper graded layer 31c. The AlGaAs lower graded layer 31a is grown on the base layer 221 to a thickness of 300 Å while gradually increasing the AlAs mixed crystal ratio from 0.1 to 0.3, the Al_0.3Ga_0.7As center layer 31b is grown on the lower graded layer 31a to a thickness of 900 Å while maintaining the AlAs mixed crystal ratio at 0.3, and the AlGaAs upper graded layer 31c is grown on the center layer 31b to a thickness of 300 Å while gradually decreasing the AlAs mixed crystal ratio from 0.3 to 0. The upper and lower graded layers 31c and 31a and the center layer 31b have the same dopant (Si) concentration of 5 x 10¹⁷ cm⁻³.
An emitter contact layer 132 comprising an n type InGaAs graded layer 32a and the n type InGaAs with super-periods GaAs layer 32b is disposed on the emitter layer 231. The n type InGaAs graded layer 32a is grown on the AlGaAs upper graded layer 31c to a thickness of 500 Å while gradually increasing the InAs mixed crystal ratio from 0 to 0.5. The dopant (Si) concentration of the InGaAs graded layer 32a gradually increases upward from 5 x 10¹⁷ cm⁻³ to 4 x 10¹⁹ cm⁻³.
The super-periods layer 32b is 500 Å thick and comprises five n type In_0.5Ga_0.5As layers 2 and four n type GaAs layers 1 which are alternatingly laminated. The thickness of each GaAs layer 1 is 10 Å, and the interval between adjacent GaAs layers 1 is about 90 Å.
A method for producing the InGaAs with super-periods GaAs layer 32b using a molecular beam epitaxy is illustrated in figure 3. In figure 3, reference numeral 120 designates a reaction chamber, numeral 121 designates a wafer supporting stage, and numeral 122 designates a heater, numeral 123 designates a shroud, numerals 124a to 124e designate knudsen cells disposed inside the shroud 123, and numeral 125 designates a shutter of the knudsen cell.
Initially, an HBT wafer 100b is mounted on the wafer supporting stage 121 and heated to a prescribed temperature with the heater 122. Then, the shutters 125 of the knudsen cells 124a, 124b, 124c, and 124d respectively containing In, Ga, As, and n type impurity Si are opened with the shutter 125 of the knudsen cell 124e containing p type impurity Be being closed, and the respective elements are evaporated and applied to the wafer in the form of molecular beams, whereby the n type In_0.5Ga_0.5As layer 2 about 90 Å thick having a dopant concentration of 4 x 10¹⁹ cm⁻³ is grown on the AlGaAs layer 231 at the surface of the wafer 100b. Then, the shutter 125 of the In knudsen cell 124a is closed and the n type GaAs layer 1 about 10 Å thick is grown on the layer 2.
Thereafter, the n type In_0.5Ga_0.5As layer 2 and the GaAs layer 1 are alternatingly grown and, when the growth of the fifth In_0.5Ga_0.5As layer 2 on the fourth GaAs layer 1 is finished, the shutters of all knudsen cells are closed. Thus, the InGaAs with super-periods GaAs layer 32b is completed. Since the InGaAs with super-periods GaAs layer 32b of this first embodiment includes no p type semiconductor layer, the shutter of the Be knudsen cell 124e is closed throughout the process.
Figures 4, 5, and 6 are graphs illustrating the characteristics of the super-periods InGaAs emitter contact layer 132 of the HBT 100 according to the present invention, in comparison with the characteristics of the In_0.5Ga_0.5As emitter contact layer 232 of the HBT 200 according to the prior art. Figure 7(a) is a sectional view of a sample 100a used for measuring the characteristics of the emitter contact layer 132 of the present invention, and figure 7(b) is a sectional view of a sample 200a used for measuring the characteristics of the emitter contact layer 232 according to the prior art.
The sample 200a comprises an i type GaAs layer 201b 2000 Å thick, an InGaAs graded layer 32a 500 Å thick (InAs mixed crystal ratio: 0 ∼ 0.5), and an In_0.5Ga_0.5As layer 32 500 Å thick which are successively grown on a LEC GaAs substrate 201. The sample 100a includes the super-periods InGaAs layer 32b shown in figure 2 in place of the In_0.5Ga_0.5As layer 32 of the sample 200a.
Figure 4 illustrates growth temperature dependence of the surface mohology, the sheet resistivity, and the deviation of the sheet resistivity of an InGaAs layer grown on a GaAs layer. In figure 4, HAZE showing the extent of roughness of the surface mohology is a value of the irregular reflectance of incident laser light standardized to incident light. As the HAZE increases, the specular surface is degraded. The sheet resistivity Rs is an inverse of the conductivity of the InGaAs layer. The deviation of the sheet resistivity in a wafer is represented by the standard deviation σ and the average value Rs, as σ/Rs.
In figure 4, , , and show the surface mohology, the sheet resistivity, and the deviation of the sheet resistivity, respectively, of the InGaAs layer 32 of the sample 200a according to the prior art, and , , and show the surface mohology, the sheet resistivity, and the deviation of the sheet resistivity, respectively, of the super-periods InGaAs layer 32b of the sample 100a according to the present invention.
As shown in figure 4, in the super-periods InGaAs layer 32b of the present invention, a favorable specular surface, i.e., HAZE ≦ 100 ppm that can be applied to an actual device, is realized at the growth temperature 450°C with no significant increase in the sheet resistivity and the deviation of the sheet resistivity. Although the super-periods InGaAs layer of the present invention appears to be inferior to the prior art InGaAs layer grown at 400°C, since it is most important to suppress the contact resistivity, the super-periods InGaAs layer is superior to the prior art InGaAs layer if the contact resistivity is taken into account (see figure 6).
Figures 8(a) and 8(b) schematically illustrate a mechanism for improving the surface mohology. In the prior art structure shown in figure 8(b), In atoms 13 are segregated at the surface of the InGaAs layer 2. On the other hand, in the structure of figure 8(a) according to the present invention, since the very thin GaAs layer 1 is present on the InGaAs layer 2, portions X of the InGaAs layer 2 where In atoms are segregated are covered with the GaAs layer 1 including no In atom and, furthermore, the lattice constant of the GaAs layer 1 is smaller than that of the InGaAs layer 2, whereby the segregation of In atoms and the elimination of In atoms from the surface are suppressed by the GaAs layer 1.
Figure 5 illustrates results of evaluation in which components of the sheet resistivity Rs is measured at different growth temperatures by Hall measurement. In figure 5, , , and respectively show the electron density, the carrier mobility, and the sheet resistivity of the InGaAs layer 23 of the sample 200a according to the prior art, and , , and respectively show the electron density, the carrier mobility, and the sheet resistivity of the super-periods InGaAs layer 32b of the sample 100a according to the present invention. The sheet resistivity Rs is represented as follows.

${Rs = ρ}_{C} (d/s)$

$ρ_{C} {= 1/(q n}_{e} µ_{e})$

where S is the area, d is the thickness of the emitter contact layer, ρ_C is the contact resistivity, q is the unit electric charge, n_e is the electron density per unit volume, and µ_e is the carrier mobility.
In figure 5, Ns is a value attained by converting the electron density per unit volume ne into the electron density per unit square. Strictly speaking, since the emitter contact layer 132 includes the graded layer 32a, a correction according to the variation in the In composition is necessary.
As shown in figure 5, the electron density and the electron mobility of the super-periods InGaAs layer according to the present invention are not very much different from those of the prior art InGaAs layer.
Since the In_yGa_1-yAs layer has smaller energy band gap and higher intrinsic carrier density than those of the GaAs layer, it is favorable when an ohmic electrode is formed thereon. Therefore, the insertion of the GaAs layer into the In_yGa_1-yAs layer adversely affects the formation of the ohmic electrode. However, the GaAs layer employed in this first embodiment is very thin so that most of the operating current can pass through that layer and, therefore, the surface mohology is improved without increasing the resistivity.
Figure 6 illustrates the contact resistivities of the super-periods InGaAs layer (emitter contact layer) at different growth temperatures in comparison with those of the InGaAs layer according to the prior art.
The resistivities are measured with respect to a three-layer structure of Ti/Mo/Au that is an emitter metal employed in an actual HBT and a single-layer structure of WSi that is a refractory metal of the highest reliability. In figure 6, and show the contact resistivities of the prior art InGaAs layer with the WSi layer and the Ti/Mo/Au layer, respectively, and and show the contact resistivities of the super-periods InGaAs layer with the WSi layer and the Ti/Mo/Au layer, respectively.
In case of the Ti/Mo/Au layer, although the average of the contact resistivities at the growth temperature of 450°C is a little higher than that of the prior art, the range of the variation of the contact resistivity is significantly reduced compared to the prior art, which means that the uniformity of the contact resistivity in a wafer is significantly improved.
In case of the WSi layer, the lowest contact resistivity between the WSi layer and the super-periods InGaAs layer according to the present invention is attained in the measuring temperature range (300°C ∼ 500°C), and the variation of the contact resistivity is suppressed to that of the prior art.
In the production of HBTs, it is very important to reduce the contact resistivity of the InGaAs layer grown on the GaAs layer with improved surface mohology and, therefore, the fact that the lowest contact resistivity is attained to the highly-reliable refractory metal, i.e., WSi, is of great significance.
As described above, according to the first embodiment of the present invention, the n type In_yGa_1-yAs emitter contact layer 32b about 500 Å thick is of the super-periods structure including four n type GaAs layers 1 each having a thickness of 10 Å, so that the surface mohology of the InGaAs layer 32b is improved with no increase in the contact resistivity, the sheet resistivity, and the deviation of the sheet resistivity in a wafer. Therefore, fine patterns can be formed on the surface of the emitter contact layer, whereby a fine emitter and a fine emitter electrode are achieved at high yield.
Further, in addition to the reduction of the contact resistivity, the uniformity of the contact resistivity in a wafer is significantly improved. Therefore, the HBT according to the present invention is very useful when a monolithic microwave IC (MMIC) is fabricated.
In the HBT with the super-periods structure, the threshold f_T of the operating frequency at which the HBT operates with a desired current amplification factor is increased to 80 GHz or more, resulting in a high-performance HBT.
Figure 9 is a sectional view illustrating an HEMT in accordance with a second embodiment of the present invention. In figure 9, an undoped GaAs layer 112 is disposed on a semi-insulating GaAs substrate 111. An n type AlGaAs layer 113 is disposed on the undoped GaAs layer 112. A two-dimensional electron gas layer (not shown) is disposed in the undoped GaAs layer 112 contacting the interface between the AlGaAs layer 113 and the GaAs layer 112. A gate electrode 116 comprising Al is disposed on a center part of the n type AlGaAs layer 113. Spaced apart n type GaAs layers 114 are disposed on the n type AlGaAs layer 113 at opposite sides of the gate electrode 116. Source and drain contact layers 132 are disposed on the respective n type GaAs layers 114. Source and drain electrodes 115a and 115b are disposed on the respective source and drain contact layers 132.
In this second embodiment, the source and drain contact layers 132 are of the super-periods structure in which four GaAs layers each having a thickness of 10 Å are inserted into an n type InGaAs contact layer at intervals of 90 Å. Since the GaAs layer is only 10 Å thick, most of the operating current passes through the layer due to the tunnel effect. The interval of 90 Å is wider than the critical thickness that maintains the pseudomorphic state of the InGaAs crystal on the GaAs crystal.
Also in this second embodiment of the present invention, similarly as in the above-described first embodiment, the InGaAs source and drain contact layer 132 can be grown at a high temperature with improved surface mohology, so that a HEMT having improved operating characteristics is produced at good yield.
The contact layer of this super-periods structure may be applied to ohmic contact layers of pseudomorphic HEMTs and MESFETs.

Claims

A semiconductor device including a lamination structure of a GaAs layer and an InGaAs layer (32b) disposed on said GaAs layer, through which operating current flows in the thickness direction of said InGaAs layer (32b);
which InGaAs layer (32b) is characterized by:
a plurality of very thin GaAs layers (1) through which most of the operating current passes due to tunnel effect, inserted into said InGaAs layer (32b) at prescribed intervals, each interval being wider than a critical thickness that maintains a pseudomorphic state of an InGaAs crystal on a GaAs crystal.
A semiconductor device including:
a lower InGaAs layer (32a) grown on a GaAs layer while gradually increasing the ratio of InAs mixed crystal from 0 to a prescribed value to a prescribed thickness more than a critical thickness that maintains a pseudomorphic state of an InGaAs crystal on a GaAs crystal;
an upper InGaAs layer (32b) grown on said lower InGaAs layer (32a) while maintaining the ratio of InAs mixed crystal at said prescribed value to a prescribed thickness more than said critical thickness, on which an insulating film or a conductive film is to be formed and through which operating current flows in the thickness direction;
which upper InGaAs layer 32(b) is characterized by:
a plurality of very thin GaAs layers (1) through which most of the operating current passes due to tunnel effect, inserted into said InGaAs layer (32b) at prescribed intervals, each interval being wider than said critical thickness.
The semiconductor device of claim 2 wherein said InAs mixed crystal ratio of said lower InGaAs layer (32a) is gradually increased from 0 to 0.5, and said upper InGaAs layer (32b) is a contact layer having the InAs mixed crystal ratio of 0.5, on which an electrode is to be formed.
A heterojunction bipolar transistor (Figs. 1 & 2) including:
a GaAs emitter layer (231);
an emitter contact layer (132) disposed on said GaAs emitter layer (231), comprising a lower InGaAs layer (32a) grown on said GaAs emitter layer (231) while gradually increasing the ratio of InAs mixed crystal from 0 to a prescribed value to a prescribed thickness more than a critical thickness that maintains a pseudomorphic state of an InGaAs crystal on a GaAs crystal, and an upper InGaAs layer (32b) grown on said lower InGaAs layer (32a) while maintaining the ratio of InAs mixed crystal at said prescribed value to a prescribed thickness more than said critical thickness;
which upper InGaAs layer (32b) is characterized by:
a plurality of very thin GaAs layers (1) through which most of the operating current passes due to tunnel effect, inserted into said InGaAs layer (32b) at prescribed intervals, each interval being wider than said critical thickness.
A high electron mobility transistor (Figs. 9 & 2) comprising:
an undoped GaAs layer (112);
an n type AlGaAs layer (113) disposed on said undoped GaAs layer (112);
an electron layer disposed in part of said undoped GaAs layer (112) contacting the interface between said undoped GaAs layer (112) and said n type AlGaAs layer (113);
a gate electrode (116) disposed on a part of said AlGaAs layer (113);
spaced apart n type InGaAs contact layers (132) disposed on said AlGaAs layer (113) via n type GaAs layers (114) at opposite sides of said gate electrode (116);
spaced apart source and drain electrodes (115a, 115b) disposed on the respective InGaAs contact layers (132);
which n type InGaAs contact layers (132) are characterized by:
a plurality of very thin GaAs layers (1) through which most of the operating current passes due to tunnel effect, inserted into each of said InGaAs layers (132) at prescribed intervals, each interval being wider than a critical thickness that maintains a pseudomorphic state of an InGaAs crystal grown on a GaAs crystal.